This invention relates to a method and apparatus for determining absolute vehicle position in vehicle navigation systems.
In vehicle navigation systems, a vehicle's position is determined through the accumulation of data gathered by various navigation sensors. Typical navigation sensors include compasses to measure the absolute vehicle heading relative to the earth's magnetic field; gyroscopes and differential odometers to measure the vehicle's relative heading; and odometers to measure the absolute distance traveled by the vehicle. Errors in vehicle position result from the accumulation of measurement errors by each of the sensors. Compass measurements are affected by magnetic anomalies such as steel bridges or buildings. Gyroscopes and differential odometers tend to have higher resolution, but their outputs are subject to drifting phenomena. As these measurement errors accumulate, the error in the vehicle position calculated by the navigation system increases.
In the past, a technique known as "map matching" has been used to correct vehicle position errors which result from the accumulation of navigation sensor errors. Map matching uses geometrical similarities in its decision making process. The navigation system compares the current vehicle trajectory to street geometries near the currently stored vehicle position. The system then corrects the vehicle position to the location which most closely matches the vehicle's trajectory.
To accomplish this, the system searches its internal map data base in the vicinity of the most recently calculated vehicle position to find street candidates which lie in the direction in which the vehicle is currently headed. The vehicle's heading, speed, and distance traveled are continuously monitored and compared to the geometry of the current "list" of street candidates. As the geometry of each street diverges from the vehicle's calculated trajectory, that street is eliminated as a possible location. This process continues until all streets are eliminated except one. That street is then stored as the current location of the vehicle.
Map matching has proven to be an effective position error correction technique in an urban environment. The nature of city streets provides a construct flow of information from the navigation sensors to the system because of the distinct character of the vehicle's trajectory. In essence, the relatively high number of significant navigation events (e.g., turns) and the short distances between such events result in a relatively accurate calculation of the vehicle's trajectory and thus a good approximation of the vehicle's absolute position.
However, map matching has been shown to be inadequate for correcting vehicle position errors on freeways and rural highways. On a freeway, for example, a vehicle can travel a great distance without the occurrence of any significant navigation events. The infrequent occurrence of the navigation events which are required for a map matching technique to make its decisions means that position corrections will rarely be calculated. Given this fact and the continuous accumulation of errors from the navigation sensors, it becomes apparent that the errors in the calculated position of the vehicle will eventually become too large for map matching to correct.
Error correction can also be achieved through the use of a global positioning system (GPS). Through the use of satellites and ground based receivers, GPS is capable of determining a vehicle's absolute position which can then be used to correct position errors made by vehicle navigation systems. However, the accuracy of GPS is dramatically affected by satellite geometry and selective availability degradation. Errors as great as a few hundred meters are not uncommon. Thus, GPS is not an entirely reliable method of position error correction.
Therefore, a method and apparatus are needed for the determination of vehicle position in vehicle navigation systems which are capable of reliably correcting vehicle position error while operating on freeways or rural highways.